1. Field of the Invention
The present invention relates to a proton conducting material, a proton conducting membrane, method for producing the same, and fuel cell using the same. More particularly, the present invention relates to a proton conducting material and proton conducting membrane, which have both strength and ion conductivity, and which are suitable for a proton conducting membrane used in a fuel cell, water electrolysis, hydrohalic acid electrolysis, brine electrolysis, an oxygen concentrator, a humidity sensor, a gas sensor and the like.
2. Background Art
A solid polymer electrolyte of a proton conducting material is a solid polymer material which has an electrolyte group such as a sulfonic acid group in the polymer chain, wherein since it can strongly bond to a specific ion and allow positive or negative ions to selectively permeate, it may be formed into particles, fiber or a membrane for use in a variety of applications such as fuel cells, electrodialysis, diffusion dialysis, and battery barrier membranes.
For example, fuel cells convert and extract the chemical energy of a fuel to direct electrical energy through electrochemical oxidation of the fuel in the cell such as hydrogen or methanol. In recent years, fuel cells have been drawing attention as a clean source of electric energy. Solid polymer fuel cells which use a proton conducting membrane as the electrolyte are in particular being anticipated as an electricity source for electric vehicles in view of the fact that they can operate at low temperatures and can achieve high output density.
The basic configuration of such a solid polymer fuel cell comprises an electrolyte membrane and a pair of gas diffusion electrodes having a catalyst layer which are coupled to both surfaces of the membrane, wherein a current collector is further provided on both sides thereof. One of the gas diffusion electrodes (anode) is provided with fuel in the form of hydrogen or methanol, while the other gas diffusion electrode (cathode) is provided with an oxidant in the form of oxygen or air, whereupon an external load circuit is connected between the two gas diffusion electrodes for operation as a fuel cell. At this time, the protons generated at the anode move towards the cathode side through the electrolyte membrane, and react with oxygen at the cathode generating water. Here the electrolyte membrane functions as a barrier membrane between the proton transporting medium and the hydrogen gas or oxygen gas. Thus, high proton conductivity, strength and chemical stability is required for this electrolyte membrane.
On the other hand, as a catalyst for a gas diffusion electrode, in general a precious metal such as platinum supported on a carrier having electron conductivity such as carbon is used. As the electrode catalyst binding agent, which channels proton movement onto the catalyst supported on this gas diffusion electrode, a proton conducting polymer electrolyte is usually used for the purpose of increasing the catalyst usage efficiency, although a fluorine-containing polymer having a sulfonic acid group, such as a perflurosulfonic acid polymer the same as the ion-exchange membrane, can also be used as this material. Here, the fluorine-containing polymer having a sulfonic acid group, which is the electrode catalyst binding agent, can also play a role as a binder for the gas diffusion electrode catalyst, or as a cementing agent to increase the adhesion of the ion-exchange membrane to the gas diffusion electrode.
In the cases of fuel cells and water electrolysis, peroxide is generated at a catalyst layer formed at the interface of the solid polymer electrolyte membrane and the electrode, and while the generated peroxide is diffusing it becomes a peroxide radical which causes degradation reactions. Therefore, it is difficult to use hydrocarbon electrolyte membranes, which are poor in oxidation resistance. For that reason, for fuel cells, generally, a perflurosulfonic acid membrane, which has high proton conductivity and high resistance to oxidation, is used.
In addition, brine electrolysis is a method for producing sodium hydroxide, chlorine, and hydrogen by electrolyzing an aqueous solution of sodium chloride using a solid polymer electrolyte membrane. In this case, because the solid polymer electrolyte membrane is subjected to chlorine and a high-temperature high-concentration aqueous solution of sodium hydroxide, hydrocarbon electrolyte membranes which have poor oxidation resistance cannot be used. Thus, for a solid polymer electrolyte membrane for brine electrolysis, generally, a perflurosulfonic acid membrane which is resistant to chlorine and high-temperature high-concentration aqueous sodium hydroxide, and further which partially incorporates a carboxylic acid group on its surface in order to prevent reverse diffusion of the generated ions is used.
However, fluorine based electrolytes as represented by a perflurosulfonic acid membrane, have very high chemical stability because they contain C—F bonds, and in addition to being used in the above-mentioned fuel cells, water electrolysis or brine electrolysis, may be used as the solid polymer electrolyte membrane for hydrohalic acid electrolysis. In addition, using their proton conductivity, they may also be widely applied to humidity sensors, gas sensors, and oxygen concentrators and the like.
As the electrolyte membrane of the fuel cell, a fluorine based membrane, having perfluoroalkylene as the backbone, and partly having an ion-exchange group such as a sulfonic acid group or a carboxylic acid group at a terminal end of perfluorovinyl ether side chains may be used. Fluorine based electrolytes such as those represented by a perflurosulfonic acid membrane, because they have very high chemical stability, are recommended as an electrolyte membrane that can be used under severe conditions. As such a fluorine based electrolyte membrane, Nafion membrane (Du Pont, registered trademark), Dow membrane (Dow Chemical), Aciplex membrane (Asahi Kasei Corporation, registered trademark), and Flemion membrane (Asahi Glass, registered trademark) and the like are known.
However, production of fluorine based electrolytes is difficult, and very expensive. Along with these problems, fluorine based electrolytes also have the drawback that they are unable to sufficiently accommodate the high temperature operation of a fuel cell.
For that reason, development of a material with ion conductivity and ion exchangeability to replace fluorine based electrolyte membranes has been desired. One of those is disclosed in the following JP Patent Publication (Kokai) No. 2001-307545 A (2001), a proton conductive membrane comprising an organic material having a backbone of a polytetramethylene oxide and a three-dimensionally crosslinked structure containing a specific metal-oxygen bond, and containing an agent for imparting proton conductivity and water in the membrane.
The three-dimensionally crosslinked structure disclosed in JP Patent Publication (Kokai) No. 2001-307545 A (2001) is a proton conducting membrane comprising an organic and inorganic materials so that while heat resistance improves due to the inorganic constituent, strength is insufficient, whereby the membrane becomes fragile, so that at the time of processing if stress is applied it will be damaged. In particular, the membrane breaks from gas pressure or shock when being used as a fuel cell. This is caused by the tensile strength and flexibility not being sufficient in the above-mentioned three-dimensionally crosslinked structure. Further still, the three-dimensionally crosslinked structure did not have sufficient proton conductivity, and especially had problems with proton conductivity at high temperatures and low humidities.
On the other hand, JP Patent Publication (Kokai) No. 5-254824 A (1993), which is directed to a layered clay mineral, discloses making a thinner membrane without damaging any of the particular structural characteristics of layered clay minerals, by continuous bonding of the layered structure of a clay mineral using the reactivity between the aluminum and phosphoric acid or phosphate group contained in the layered clay mineral. JP Patent Publication (Kokai) No. 5-254824 A (1993) further discloses the production of a layered clay mineral thin membrane which bonds unit structures with a phosphate group, by developing on a substrate a developing liquid which contains a compound having a phosphoric acid group and a layered clay mineral comprising an aluminum backbone, then removing solvent from the liquid membrane on the substrate.
However, the layered clay mineral thin membrane disclosed in Patent Document 2 has low ion exchangeability, and its function when used as an ion-exchange membrane in a fuel cell is considerably weak. This is because the functional group governing ion-exchange is only the phosphate group crosslinking the clay mineral, so that compared with a conventional fluorine based ion-exchange membrane, ion exchangeability is low. Further, in addition to this layered clay mineral thin membrane having an insufficient proton conductivity, it has problems such as insufficient strength, insufficient flexibility (weak against deformation), insufficient size stability when swollen (water absorption) (in a restricted state size would change in the cell, causing it to burst), and uncontrollable gas permeability (for membranes shielding properties are important, and for a catalyst layer electrolyte permeability is important).
Therein, there was a need to develop a new proton conducting membrane and proton conducting material to replace the perfluorocarbonsulfonic acid based proton conducting membrane which was generally used for solid polymer electrolyte type fuel cells (PEFC).
It is an object of the present invention to solve the above problems of a conventional proton conducting material and proton conducting membrane, by providing a proton conducting material and proton conducting membrane with, in particular, high strength, flexibility (strong against deformation), and high size stability when swollen (water absorption). It is a further object of the present invention to realize a high performance fuel cell which uses these.